Dry eye syndrome (DES) results from an inadequate tear film on the corneal surface. As a result, patients with DES suffer from ocular pain and in some cases serious vision problems. DES affects as much as 20% of the population and treatment often remains inadequate. One cause of dry eye may be an inability of sensory neurons to properly assess the ocular surface fluid status by responding to drying of the cornea. The neural regulatory mechanisms involved in maintaining a normal tear-film, via basal tear production, are still unknown. We have recently discovered that a class of corneal primary afferent neurons, cold cells, which are known to respond to non-noxious cooling, are also activated by drying of the ocular surface, hyperosmotic solutions, and menthol. These properties would be expected of neurons that participate in the regulation of basal tearing, as evaporation of tears from the corneal surface causes both cooling and increased osmolarity of the tear film. It is hypothesized that these dry-responsive cold cells represent the afferent limb of the reflex arc that drives non- noxious basal tearing without producing ocular pain. This hypothesis will be examined in four specific aims. The first aim will determine the encoding properties of corneal primary afferent neurons activated by drying of the ocular surface. A particular focus will be on hyperosmotic and TRPM8 dependent evoked responses; TRPM8 channels are activated by both menthol and innocuous cooling. Corneal primary afferents will be characterized using in vivo single-unit electrophysiology in the rat trigeminal ganglion. Responses to drying of the ocular surface will be examined, as well as responses evoked by thermal, chemical, and osmotic stimuli. Aim 2 will determine the ability of TRPM8 agonists to alter cold- and dry-evoked responses in corneal primary afferent neurons. Aim 3 will determine the encoding properties and projection targets of spinal trigeminal nucleus (Vsp) neurons activated by drying of the ocular surface. Single unit recordings in Vsp will be used to assess properties of neurons that receive direct input from corneal primary afferents, and electrical stimulation will be employed to test for their projection status to regions involved in the regulation of tearing. Finally, Aim 4 will determine the circuitry within the spinal trigeminal nucleus involved in lacrimation and nocifensive behaviors evoked by TRPM8 and TRPA1 agonists applied to the ocular surface in rats. Lesions in two distinct regions of Vsp that receive corneal inputs will be performed to determine their relative contribution to tearing and nocifensive responses to ocular stimulation. The ability to increase the sensitivity and activity of neurons that respond to drying of the ocular surface represents a novel strategy for the potential treatment of DES.